The rules for a Druid's Wild Shape ability say "When you transform, you assume the beast’s hit points and Hit Dice." But the stat block for "A monster's hit points are presented both as a die expression and as an average number."
Now, for your typical monster that the DM controls, the DM can of course choose whether to just take the average or actually roll for how many hit points it should have. But here, it's a player that's using the monster's stat block.
So, does the player choose whether to take the average or to roll for hit points for their new form? Do they need to roll because the ability to just take the average is a DM-only thing? Do they need to just take the average because that's the standard "monster statistic"? Or does the DM pick each time just like they'd pick for a monster, even though it's for a player?
I'd prefer official sources or references if available. If there aren't any, then I would accept any semi-official or well-informed well-reasoned arguments, preferably backed up by whatever evidence is available. Obviously as with any rules interpretation the DM has the final say, but I'm usually the DM and I'm not sure what to do.